Genes in teats and placentas
What have a placenta and a marsupial mammary gland got in common? When scientists in America and in Australia compared the patterns of genes active in both, the answer is quite a lot. Chris Smith speaks with Marilyn Renfree, from the University of Melbourne…
Marilyn: A marsupial is a mammal. Like all mammals, it has hair and it lactates, but marsupials have been separated from other mammals for about 160 million years. They probably evolved in China, in Asia, there's lots of amazing fossils coming out of China. There's very few differences really between eutherian mammals and marsupial mammals except in their mode of reproduction. All marsupials give birth to tiny babies that then spend a long time – instead of in the uterus, they spend it in the pouch. So, a long time ago, I said that I thought that marsupials had traded the umbilical cord of the placenta for the teat because lactating is very sophisticated in marsupials. They're well-known for their amazing lactational physiology.
Chris: It might be a surprise to many people to hear you talking about placentas in the same sentence as talking about the word ‘marsupial’ because most people are of the opinion that tiny baby emerges, it goes on this teat in the pouch and that placentas have nothing to do with the equation.
Marilyn: Sadly, that’s an era that was made has been propagated in the public and in the literature for a long time where people just assume that marsupials have no placenta. That’s completely wrong and the placenta is the most variable structure in the animal kingdom. Every group of mammals has a different placenta that has slightly different structures but all of them function to transfer food and immune material across to the foetus to protect it. So there's really two ways – oviparous mammal, the mammal that gives birth to live young can protect their young. One is to keep them for a long time in the uterus which is what most eutherian mammals do and the other is to keep them for a long time in a pouch or a nest which is what marsupial mammals do.
Chris: So how have you explored this question that the umbilical cord has been traded for a teat in the marsupials then?
Marilyn: My fantastic colleagues at Stanford, they were able to use their fantastic facilities to look at the genes that are expressed in both placenta and in the mammary gland of marsupials or our favourite marsupial, the tammar wallaby and compare it to what happens in the mouse. Of course, there are many differences, but in the genes that Julie Baker and co. examined, they found a number of genes that were expressed in the placenta that were also expressed in the marsupial mammary gland. In particular, one called GCM1 and which is essential to eutherian placentation is in the mammary gland of marsupials. This is the first time this particular gene was found to be expressed outside the placenta, suggesting that this trade-off between umbilical cord and teat has been quite widespread and very effective in this very, very successful mode of reproduction.
Chris: Are there any other spin-offs from this study? Apart from insights into evolution, does it inform other aspects of how mammals work and reproduce?
Marilyn: Yes, I think it does because marsupial lactation is the most sophisticated in the mammal kingdom and marsupial milk changes dynamically through the whole of lactation. So in the case of the tammar wallaby, it’s a 9 or 10-month lactation period and the early milk is very high in carbohydrate so it’s sweet and very low in fat whereas the late milk is very high in fat and low in carbohydrates, and has specific proteins. So early milk is like human milk and late milk is like cow’s milk. What do we feed our infants but humanised formula of cow’s milk? In our studies showing transfer of young from an early stage of lactation where the milk is dilute to a late stage of lactation where the milk is high fat, we get growth acceleration and a lot of accumulation of fat around every organ of the body except for the brain. So we’re wondering whether this is giving us a clue to the early neonatal reprogramming that we do that may well affect later onset human obesity.